import numpy as np

def change(lst):
    np.sign.at(lst, [2, 4])

def test_change():
    np.random.seed(44)
    arr = np.random.randint(-4, high = 4, size = 7)
    sign2 = np.sign(arr[2])
    sign4 = np.sign(arr[4])
    change(arr)
    assert sign2 == arr[2]
    assert sign4 == arr[4]
